Field of the Invention
The description relates to the field of image sensors. More specifically it relates to a device and method for providing a high dynamic range digital readout of at least one pixel of an image sensor.
Description of the Related Technology
An image sensor device may typically involve AD (analog-digital) conversion of a signal generated by semiconductor elements for detecting a physical quantity distribution, e.g. a radiative quantity such as a light intensity of incident light on a focal plane of the image sensor. Such image sensors may typically detect a distribution of a radiative quantity, such as photons, electrons or protons incident on the pixels. For example, such device may comprise a plurality of semiconductor elements arranged in an array which are sensitive to electromagnetic waves received from the environment, such as light or other types of photon radiation. Furthermore, methods for digital signal processing may be used for reading out and converting the physical quantity distribution into a suitable signal representation, e.g. a digitized electric signal. Image sensors with integrated ADCs (analog to digital converters) may typically apply a quantization of the analog pixel output signals to the digital domain.
A plurality of pixels may be logically arranged in rows and columns in an imaging device, according to a device design known in the art. For example, a pixel signal may be read out through a column parallel output arrangement. In such an address control system, typically one row in the pixel array or a sub-array thereof may be selected for concurrent access, such that the pixels in this row can be processed simultaneously and in parallel through column readout circuitry. For example, a column line may direct the pixel outputs to the readout circuitry, which may comprise an ADC for quantizing this signal. In this manner, a single row of pixels may be read out by selecting the pixel corresponding to this row in each column and processing all signals from this row in parallel by the dedicated readout circuitry of each column.
With the advance of technology, image sensors have become faster, while their pixel size keeps decreasing. For example, various problems associated with traditional charge coupled device (CCD) imaging sensors, e.g. which limited acquisition speed and pixel size, have been overcome by Complementary Metal-Oxide Semiconductor (CMOS) processing techniques. These techniques allow electrical signal amplification on the pixel level, e.g. in Active Pixel Sensors (APS).
For example, the achievable resolution of quantization of such devices as known in the art may have increased to more than 14 bit, the noise may have reduced to one electron readout noise or even less on average and the capture speed may have increased to more than 1000 frames per second. At the same time, the number of pixels is dramatically increased to for example more than 10 million.
This poses high demands on the ADC, which needs to be fast and provide low noise and high dynamic range quantization. Moreover, the ADC should preferably be small and have low power consumption, since thousands of such ADCs may be present on an imager chip.
These ADC requirements are stringent. However, for a large analog pixel output signal provided to the ADC as input signal, the resolution of the ADC may be less critical than for a small analog pixel output signal. For example, photon shot noise in the pixel, which may be substantially proportional to the square root of the signal, can be large for large signals, making high resolution and low-noise at such large signal not needed. However, typically ADCs or readout systems do not exploit this property.
One method known in the art for advantageously exploiting this property involves using a slope ADC with variable slope. However, slope ADCs may have the disadvantage of being relatively slow and may provide a relatively noisy readout compared to other AD conversion techniques.